$\dfrac{6}{12} - \dfrac{3}{8} = {?}$
$ = {\dfrac{6 \times 2}{12 \times 2}} - {\dfrac{3 \times 3}{8 \times 3}} $ $ = {\dfrac{12}{24}} - {\dfrac{9}{24}} $ $ = \dfrac{{12} - {9}}{24} $ $ = \dfrac{3}{24}$